In the known prior art, modulation doped semiconductor structures show interdigitated comb gates or grids which modulate the carrier concentration of a two-dimensional electron gas which is closely coupled to a conducting layer. In an article entitled "Heterostructure Traveling Wave Transistor" by Fang et al, IBM Technical Disclosure Bulletin, Vol 31, No. 8, January 1989, pages 150-152, an interdigitated comb gate disposed on a modulation doped structure modulates the carrier concentration of a two-dimensional electron gas which is closely coupled to a conducting layer. The latter has a pair of spaced apart contacts to which an electric field is applied. When an AC signal is applied to the terminals of the gates, a periodic variation in the carrier density of the two-dimensional electron gas is obtained resulting in a longitudinally polarized standing wave. If the carrier relaxation time is long relative to the transit time between the fingers of the interdigitated gate electrode, the carriers will tend to bunch spatially with the same periodicity as that of the fingers. With a potential applied between the contacts to the closely coupled conducting layer, the carriers therein move and form a traveling space charge wave. When the velocity of the space charge wave is about equal to the wave velocity established by the AC signal applied to the gates and the drift velocity (ME) is larger than the wave velocity (f.lambda.), there will be a strong coupling and energy transfer between the gate field and the electrons in the closely coupled layer resulting in signal amplification. In the arrangement of the reference, there is no superlattice effect involved because the elements involved are on too large a scale. Also, it should be noted that the interdigitated comb is not displaced from a current path so there is effectively no confinement of current which is quasi-one-dimensional in character. Also, in the present application, there is no need to apply AC potentials to the gate to obtain oscillatory behavior.
In another article entitled "Negative transconductance and negative differential resistance in a grid-gate modulation-doped field-effect transistor" by Ismail et al, Appl. Phys. Lett. 54 (5), 30 Jan. 1989, page 460, a grid-gate lateral-surface-superlattice (LSSL) field effect transistor in the modulation doped device environment is shown. The grid-gate provides a tunable, two-dimensional periodic potential modulation to electrons traveling from source to drain. This arrangement does not provide a preferential current path in that potential barriers in two-dimensions are interposed in the LSSL device directly in the current path. There is no lateral action at-a-distance which quantum mechanically affects carriers preferentially confined to a given current path so that the carriers act as if they have actually encountered superlattice structure but without the disadvantages of such heterojunction devices. In addition to providing a quasi-one-dimensional current path, the present approach also eliminates free electron behavior permitting the formation of true minigaps.
In an article entitled "Conductivity oscillation due to quantum interference in a proposed washboard transistor" by Tokura et al, Appl. Phys. Lett, 51 (22), 30 Nov. 1987, page 1807, a grating gate or washboard is shown disposed on the surface of a modulation doped device. The two-dimensional electron gas formed in the device is modulated by a weak periodic potential producing a conductivity oscillation due to the electron velocity modulation caused by a quantum interference effect. To the extent that the periodic potential applied to this structure extends from edge-to-edge of the structure, any current flow in a direction perpendicular to the elements of the washboard will be spread over the width of the device with no preferential current path. Thus, carriers will encounter induced potential barriers as they travel through the washboard device. Because there is no preferred current path inelastic scattering as well as loss of phase coherence can occur resulting in device characteristics degradation. The approach of the present application avoids such degradation by displacing the means which periodically interrupt the two-dimensional electron gas from a given current path so that current flow is provided with a preferential narrow current path. Displacing the interrupting means from the current path also quantum mechanically affects the carriers at-a-distance in such a way that the carriers act as if they had encountered a heterojunction type superlattice with all its associated advantages and none of its disadvantages.
An article entitled "Quasi-One-Dimensional Channel GaAs/AlGaAs Modulation Doped FET Using a Corrugated Gate Structure" by Okada et al, Jap. Journal of Appl. Phys. December 1988, 27 L2424-L2426, shows a modulation doped structure with a corrugated gate structure in which the electron gas confinement has been changed from two-dimensional to quasi-one-dimensional with a negatively biased gate voltage. Enhanced field-effect mobility and transconductance oscillations in a strictly confined one-dimensional channel regime were seen. In the article, a modulation doped structure is covered with a plurality of semiconductor stripes over which a gate electrode is conformally deposited forming a gate for two-source-drain pairs. A two-dimensional carrier gas (2DEG) is formed when the gate bias is zero. When negative bias is applied, the 2DEG becomes a plurality of quasi-one-dimensional gas regions, because the 2DEG is depleted in the regions without the stripes. With the change in bias, the transverse current disappears, but the longitudinal quasi-one-dimensional current remains. In the arrangement of the reference, the corrugations involved are placed directly in the current paths between the source/drain pairs and with zero bias. The 2DEG provides a current path between them.
With the application of bias, the 2DEG is partitioned into channels which extend between one source/drain pair. In this way, current flows between the one source/drain pair but not the other. There is therefore, no current flow in a direction perpendicular to the direction of the stripes in the presence of bias on the gate as there is in the embodiments of the present application where current flows in a quasi-one-dimensional channel when confined by a plurality of protuberances and indentations formed in gates or in the modulation doped structure itself. There doesn't appear to be any intention to fabricate a superlattice-like structure in which carriers encounter superlattice-like effects due to 2DEG interruption means disposed along and displaced from a current path.
U.S. Pat. No. 4,758,868, originally filed Jan. 3, 1983, shows in FIG. 1 an embodiment of a permeable base transistor which consists of an ohmic emitter contact on an n+ layer of gallium arsenide. A pair of insulating regions confine current flowing from the contact past a gate made of a plurality of fingers to a collector. In this arrangement, current flow is perpendicular to the plane of the finger rather than being in a plane parallel to the plane of the corrugations as in the device of the present application. In the present application, a quasi-one-dimensional gas is confined by the means for interrupting portions of the 2DEG such that deleterious behavior like scattering from heterojunctions is eliminated.
It is, therefore, an object of the present invention to provide a quantum mechanical effect device in which superlattice effects are experienced by carriers without the use of known superlattices with their alternating layers of different materials and associated interfaces.
Another object of this invention is to provide a quantum mechanical effect device in which heterojunction interfaces are eliminated while simultaneously providing superlattice-like effects on carriers.
Yet another object is to provide a quantum mechanical effect device wherein superlattice-like effects are produced in the modulation doped regime by periodically interrupting the two-dimensional carrier gas produced therein with means displaced from a current path which maintain or deplete portions of the carrier gas in a periodic fashion.
Still another object is to provide a quantum mechanical effect device in which inelastic scattering and loss of phase coherence due to the presence of discontinuities is substantially eliminated.
Another object is to provide a quantum mechanical effect device wherein available carriers are confined in a quasi-one-dimensional gas by the action of a plurality of protuberances and indentations strung along but displaced from the current path while being simultaneously subjected to superlattice-like effect.
These and other objects, features and advantages will become more apparent from the following more particular description of the preferred embodiments.